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Fine-tooth combing

the Universe BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 05, 2008

A Nobel Prize-winning technology – a laser frequency ‘comb’ – could bring astronomers a step closer to answering crucial questions about the expanding Universe, and help in the search for extrasolar Earthlike planets.

"It looks as if we are on the way to fulfill one of astronomers'
dreams," says team member Theodor Haensch, director at the Max Planck Institute for Quantum Optics (MPQ) in Germany, and recipient of the 2005 Nobel Prize in Physics with colleague John Hall, for work including the frequency comb technique.

The invention is based on a key piece of astronomical instrumentation: the spectrograph, a device used to split light from celestial objects into its component colours, or frequencies, which can then be used to derive velocities of stars, galaxies and quasars, search for planets around other stars, or study the expansion of the Universe. But a spectrograph must be accurately calibrated so that the frequencies of light can be correctly measured, just like accurate rulers are needed to measure lengths correctly. The laser comb provides the most accurate ruler ever, albeit to measure colours rather than distances.

Artist impression of the European Extremely Large Telescope (E-ELT). The primary mirror has a diameter of 42 metres. Image: ESO.

Future telescopes, such as the European Extremely Large Telescope (E-ELT) will require even more precise spectrographs than are available now, which in turn will need to calibrated with even more accurate ‘rulers’. In fact, they must be accurate to about one part in 30 billion, equivalent to measuring the circumference of the Earth to the accuracy of one millimetre.

"We'll need something beyond what current technology can offer, and that's where the laser frequency comb comes in,” says PhD student and team member Constanza Araujo-Hauck from ESO.

The new calibration technique was born from a collaboration between astronomy researchers at ESO and quantum optics experts at MPQ and uses ultra-short pulses of laser light to create a 'frequency comb'– light at many frequencies separated by a constant interval – to create just the kind of precise 'ruler' needed to calibrate a spectrograph.

After successful tests in the MPQ laboratory in 2007, the team subsequently tested a prototype device using the laser comb at the Vacuum Tower Telescope, a solar telescope in Tenerife, to measure the spectrum of the Sun in infrared light. Following that success, a new version of the system is now being built for the HARPS planet-finder instrument on ESO's 3.6 metre telescope at La Silla in Chile, before being considered for future generations of instruments.

The new laser comb technology could give astronomers the precision they require to detect Earthlike planets around other stars. Image: ESO.

One of the most ambitious projects to be realised with the E-ELT – CODEX, a high-resolution visual spectrograph – aims to directly measure the acceleration of the Universe by charting the velocities of distant galaxies and quasars over a 20 year period. This would let astronomers test Einstein's theory of general relativity and probe the nature of the mysterious dark energy component of our Universe.

"We have to measure the movement of these distant galaxies to a few centimetres per second, and follow this over decades,” says Antonio Manescau from ESO. “These speeds are barely faster than a snail's pace, and the laser frequency comb is absolutely crucial for this."

The laser comb will also help astronomers in the hunt for planets around other stars, where astronomers often make use of spectrographs to watch for subtle movements of a star as the planet orbits it. To be detected with current technology, these planets must be relatively massive or close to the star compared to Earth. A more precise spectrograph could see astronomers finding planets outside of our Solar System with characteristics similar to those of the Earth, the holy grail of extrasolar planet hunting.

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